1. Field of the Invention
The present invention relates to an optical fiber and a method of manufacturing the same. The present invention can be applied to a 1.3-xcexcm single-mode ribbon fiber, a dispersion-shifted fiber, and a dispersion-compensated fiber, as well as any other types of optical fibers and their manufacturing methods, and is particularly suitable to a dispersion-compensated fiber having a high Ge content in its core, concretely, a dispersion-shift fiber (DSF) and a dipsersion-flat fiber (DFF) having a GeO2 content at about 0.5 to 1.0% in their cores, and a dispesion-compensation fiber (DCF) having a GeO2 content at about 1 to 3% in its core and a fiber for an optical amplifier and large PMD (Polarization Mode Dispersion: also called merely xe2x80x9cpolarization dispersionxe2x80x9d), and a method of manufacturing the same.
2. Related Background Art
In a conventional optical fiber manufacturing method in which one end of an optical fiber preform is softened by heating and an optical fiber is drawn from it, it is difficult to make the core portion of the optical fiber and a cladding portion around the core portion to have perfectly circular and concentric sections, and the sections of the core portion and cladding portion usually become slightly elliptic or slightly distorted circular. Accordingly, the refractive index distribution in the sectional structure of the optical fiber is not completely uniform, which causes a difference in group velocity of two orthogonally polarized waves in the section of the optical fiber, thereby undesirably increasing polarization dispersion. For this reason, when the optical fiber is put into a practical use as a submarine cable or trunk cable that require large-capacity, long-distance transmission, the adverse influence of the polarization dispersion appears largely. Even in optical fibers having almost the same diameter, the higher the content of the dopant, e.g., GeO2, added to the core, the larger the polarization dispersion.
It is an object of the present invention to effectively impart a predetermined twist to an optical fiber by rolling the optical fiber on the roller surface of a guide roller in accordance with the swing motion of the guide roller, thereby providing an optical fiber in which, even if the sectional shapes of its core portion and cladding portion are not perfectly circular and concentric, polarization dispersion can be suppressed in the elongated optical fiber as a whole equivalently as in a case wherein the sections of the core portion and the cladding portion are perfectly circular and concentric, and a method of manufacturing the same.
The present invention relates to an optical fiber manufacturing method comprising the first step of drawing an optical fiber from an optical fiber preform, the second step of coating the optical fiber with a predetermined coating material, and the third step of imparting a predetermined twist to the optical fiber coated with the predetermined coating material. The third step further comprises the first substep of guiding the optical fiber coated with the predetermined coating material with a first guide roller that swings periodically, and rolling the optical fiber on a roller surface of the first guide roller in accordance with swing of the first guide roller, and the second substep of guiding the optical fiber that has passed through the first guide roller with a second guide roller provided to a next stage of the first guide roller and having a fixed rotating shaft, and suppressing the optical fiber from rolling on a roller surface of the second guide roller with an optical fiber rolling suppression means provided to the second guide roller.
The optical fiber rolling suppression means provided to the second guide roller is preferably a V-shaped, U-shaped, or convex narrow groove which is formed in the roller surface of the second guide roller to fit the optical fiber in it.
It is preferable that the outer diameter and position of each of the first and second guide rollers be adjusted so that a length with which the optical fiber contacts the roller surface of the first guide roller is substantially equal to or less than a roller circumference corresponding to a central angle of 90xc2x0 of the first guide roller.
It is preferable that the roller surface of the first guide roller with which the optical fiber contacts be covered with a resin having a high coefficient of friction against the predetermined coating material of the optical fiber.
The resin to cover the roller surface of the first guide roller is preferably an urethane resin or an acrylic resin.
The optical fiber preferably has a drawing tension of 4.0 kg/mm2 or more and 16 kg/mm2 or less.
The third step preferably further comprises the substep of suppressing responsive motion of the optical fiber, which is caused by swing of the first guide roller, with an optical fiber responsive motion suppressing means provided on a preceding stage of the first guide roller.
It is preferable that the optical fiber responsive motion suppressing means be at least a pair of guide rollers which are provided above the first guide roller at a predetermined distance to oppose each other at a predetermined gap through which the optical fiber is passed.
Further, it is another object of the present invention to provide an optical fiber drawing method for reducing polarization characteristic of an optical fiber, comprising the steps of: drawing the optical fiber from an optical fiber preform; coating the optical fiber with a predetermined coating material; and guiding the optical fiber coated with the predetermined coating material with a first guide roller that swings periodically, and rolling the optical fiber on a roller surface of the first guide roller in accordance with swing of the first guide roller; and guiding the optical fiber that has passed through the first guide roller with a second guide roller provided to a next stage of the first guide roller and having a fixed rotating shaft and suppressing the optical fiber from rolling on a roller surface of the second guide roller with an optical fiber rolling suppression portion provided to the second guide roller, wherein the swing of the first guide roller is such that the maximum clockwise angle and the maximum counterclockwise angle of the first guide roller are equal, that a period of the clockwise swing which is the time from the beginning to the end of the clockwise swing of the first guide roller and a period of the counterclockwise swing which is the time from the beginning to the end of the counterclockwise swing of the first guide roller are equal, and that a swing direction of the first guide roller is reversed smoothly without stopping when the swing angle of the first guide roller becomes maximum.
It is preferable in the above method that an optical fiber responsive motion suppressing means is provided to a preceding stage of the first guide roller, the means comprises at least one pair of responsive motion suppressing rollers which are provided above the first guide roller at a predetermined distance, the responsive motion suppressing rollers opposing each other at a predetermined gap through which the optical fiber is passed, and direction of a roller shaft of one of the at least one pair of guide rollers is perpendicular to a direction of a rolling shaft of the other of the two pairs of guide rollers.
It is preferable that width of the roller surface of the first guide roller on which the optical fiber can be rolled, is not less than 3 mm.
It is preferable that the first guide roller has flanges at both sides of the roller surface, and the optical fiber does not come into contact with the flanges when the swing angle of the first guide roller becomes maximum.
It is more further object of the present invention to provide an apparatus used by an optical fiber drawing method comprising: a base; a pinion gear rotatably supported about a shaft by the base, the shaft being perpendicular to the rotating shaft of the first guide roller and passing through substantially center of the first guide roller; a rack gear meshing with the pinion gear; a first movable portion linearly moving as against the base in a direction perpendicular to a direction of the rotating shaft of the pinion gear, the rack gear being fixed to the first movable portion; a second movable portion linearly moving as against the first movable portion is a direction perpendicular to both direction of linear moving of the first movable portion and the direction of the rotating shaft of the pinion gear; and a motor rotating at constant velocity and giving a rotating motion with constant velocity as against the base to the second movable portion.
It is preferable in the above apparatus that materials of the rack gear and the pinion gear are different from each other.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.